What I suggest for people taking my survival courses are along the following lines for a day pack:
Knife (“no knife, no life”), being sure it’s either fixed blade or has a good lock. Folding knives with bad or no locks are not worth it.
Multitool.
Mylar emergency blanket and/or tarp.
Extra socks and hats.
Water bottles that are stainless steel, especially if you don’t also bring a small cooking pot.
Collapsible or otherwise quite small cook set.
Basic first aid kit.
550 paracord.
Some sort of firestarter that you’re comfortable with. Then, two more.
Medication, if you require any.
Spare glasses, if you require any.
Flashlight.
Poncho.
Backup food isn’t essential, but quite helpful, particularly if you’re not used to going without. I think there is more danger from people getting irritable and not thinking clearly than there is of actual starvation.
Less essential suggestions:
Mylar “bivvy” bag.
Small hatchet or machete.
Folding saw.
Trowel or small shovel.
Wool blanket.
U.S. Army survival guide, or something similar.
Chapstick.
Small gauge steel wire, preferably dull green craft wire (for making snares).
Fish hooks and line.
Duct tape.
Radio.
Compass.
Ceramic water filter.
Scarf.